1. Technical Field
The subject matter herein relates to analgesic compositions, formulations, and methods of use and to synergistic combinations of neurotensins or analogs and opiates or analogs that reduce their side effect profile at the same or increased analgesic potency.
2. Background Information
Analgesics are used in the treatment of pain, the cause of which can vary from acute wounds such as cuts, bruises, surgical incisions, or burns to chronic conditions such as structural defects (e.g., back, joint, or spinal disc problems) or diseases (e.g., cancer, inflammatory conditions, or infections).
Analgesics have an ability to reduce perception of pain impulses by the central nervous system. Opiates are the most widely used analgesics in the treatment of severe pain. Opiates bind a variety of receptors, including mu, delta, and kappa receptors. Both the endogenous opioid peptides and synthetic opiate analgesics alter the central or peripheral release of neurotransmitters from afferent nerves sensitive to noxious stimuli. The specific actions of the narcotic analgesics can be defined by their sensitivity and selectivity for binding at three specific opiate receptor types, mu, kappa, and delta. The mu opiates have high affinity and selectivity for binding sites in the brain and analgesic action is mostly attributed to these receptors. Delta receptors are mostly located in the spinal cord and may have a role in peripheral pain. Kappa receptors are located in the hypothalamus and may account for the neuro-endocrine actions associated with kappa binding.
Narcotic analgesics are also classified as agonists, mixed agonist-antagonists, or partial agonists by their activity at opiate receptors. Narcotic agonists include natural opium alkaloids (e.g., morphine, codeine), semisynthetic analogs (e.g., hydromorphone, oxymorphone, oxycodone), and synthetic compounds (e.g., meperidine, levorphanol, methadone, sufentanil, alfentanil, fentanyl, remifentanil, levomethadyl). Mixed agonist-antagonist drugs (e.g., nalbuphine, pentazocine) have agonist activity at some receptors and antagonist activity at other receptors; partial agonists (e.g., butorphanol, buprenorphine) are also included. Narcotic antagonists (e.g., naloxone) do not have agonist activity at any of the opiate receptor sites. Antagonists block the opiate receptor, inhibit pharmacological activity of the agonist, and precipitate withdrawal in dependent patients.
When given acutely in the treatment of severe pain, e.g., in post-op pain, opiates produce a variety of secondary pharmacological side-effects, ranging from mild to life threatening. Cough and respiration are depressed, and fatal doses lead to severe respiratory depression by direct inhibition of the respiratory center in the brain stem. Nausea and vomiting occur in many individuals through direct stimulation of the chemoreceptor trigger zone. Therapeutic doses also result in inhibition of baroreceptor responses and hypotension, the latter through the release of histamine. Gastrointestinal motility is reduced, resulting in constipation. Sedation occurs and cognitive function is impaired. Extended use of opiates, as in the treatment of cancer pain, is associated with dependence, tolerance, and potential for drug abuse.
Neurotensin and its analogs are also potent analgesic in animals. Like opiates, they are produced in the brain, spinal cord dorsal horn, hypothalamus, and gut. In all these locations, cells producing neurotensin are in close proximity to those producing endogenous opiates, which is consistent with the fact that neurotensin and opiates have similar actions. Several different neurotensin receptors (NTRs, e.g., NTR1, NTR2, and NTR3) have been identified to date, presumably with slightly different functions. Several similarities exist in the actions between neurotensin and opiates. First, neurotensin receptors involved in the treatment of central pain may be different than those involved in the treatment of peripheral pain. Second, neurotensin administration is associated with not just analgesia but hypotension (unrelated to histamine release), fall in basal temperature, and weight loss. Third, neurotensin induces tolerance. However, unlike opiates, neurotensin does not depress respiration, suppress coughing, induce constipation, alter cognitive function or cause sedation. Neurotensin is known to increase gastrointestinal transit and induce diarrhea. Neurotensin has also been shown to exhibit antipsychotic effect and antiparkinsonian effect.
To minimize side effects generated by giving a specific opiate, different opiates can be combined to produce synergistic analgesic effects. Synergism is defined as correlated action of two or more agents so that the combined action is greater than the sum of each acting separately. For example, when morphine and methadone are combined, analgesic synergy is achieved but not accompanied by synergistic effect in other pharmacological actions, such as in gut motility. This synergy between the opiates is useful because unwanted side effects associated with both acute and long-term administration of the opiates can be diminished without reducing analgesic potency. However, synergistic action between different opiates remains unclear because two opiates acting on the same receptors, namely mu receptors, may not exhibit synergy. For example, methadone is only synergistic with morphine, codeine, 6-acetyl morphine, and morphine-6-beta glucuronide but not with fentanyl, oxymorphone, oxycodone, meperidine, or alfentanyl. All of the above mentioned opiates are mu receptor agonists. Similarly, morphine is synergistic only with methadone but none of the other mu agonists.
Opiates remain the drug of choice in management of severe pain to date. However, new agents and methods are needed to provide alternative pain management, with or without opiates. Alternative agents and methods are needed to enhance the pharmacological effect and minimize unwanted side effects of opiates, such as tolerance, dependence, and constipation.